Photosensitive films comprising silver halides have been a primary object of photographic research. Although the photolytic reduction of halides to provide the latent silver photographic image is of major interest, the reverse reaction through which metallic silver is reconverted to a silver halide by the action of light or heat has also been the subject of study.
An early discussion of the changes in absorption behavior produced in a darkened photographic plate by exposure to red light is provided by Cameron and Taylor in "Photophysical Changes in Silver-Silver Chloride Systems," Journal of the Optical Society of America, Vol. 24, pp. 316-330 (1934). These authors verified that optically or chemically darkened silver halide-containing emulsions can be selectively bleached, particularly with red light, such that they become more transparent to light of the bleaching wavelength. This behavior is referred to as color adaptation. It was further noted that polarized bleaching light produced a dichroic, birefringent image in the darkened film.
Recently, it was discovered that color adaptation, dichroism and birefringence could be optically induced in certain colored glasses containing silver halides by bleaching the glass with polarized light. As described by Araujo et al. in a copending, commonly assigned patent application, Ser. No. 739,205, filed Nov. 5, 1976, glass containing an additively colored silver halide phase, when irradiated with polarized light, typically becomes selectively bleached in a manner providing increased transparency with respect to light of the same polarization and color as the bleaching light. Thus the glass exhibits dichroism, birefringence and color adaptation which depend on the color and direction of polarization of the light used to bleach the glass, and information concerning this light can be deduced by examining the glass, as long as the bleached image persists.
As used in the prior art and in the present description, the term "additive coloration" refers to coloration caused by the presence of light-absorbing metal particles in a halide crystal of the same metal. Thus additively colored silver chloride is silver chloride wherein metallic silver particles are present in or on the silver chloride crystals.
Optically-induced dichroism has also been observed in silver-containing polycrystalline silver halide layers produced by evaporation techniques. Dichroism induced by bleaching silver halide films containing additions of vacuum-evaporated silver was reported by V. P. Cherkashin in Soviet Physics-Solid State, Vol. 13, No. 1, pp. 264-265 (1971). In the Russian journal Opt. Spektrosk, Vol. 40, pp. 1024-1029 (June 1976), L. A. Ageev et al. describe dichroic effects which were observed in silver/silver halide films produced by depositing a thin granular layer of silver on a glass substrate and then converting part of the silver to silver iodide by treatment in an iodine atmosphere.
In our copending patent application, Ser. No. 739,121, filed Nov. 5, 1976 and commonly assigned herewith, we describe multilayer photosensitive films comprising discrete metal island layers disposed between layers composed of a clear dielectric acceptor material such as AgCl, PbI.sub.2 or the like. These films are light-absorbing films which can be bleached with visible light, and are useful for storing information relating to the intensity, polarization and, particularly, the color of bleaching light.
Silver halide layers also comprise important elements of many photochromic films, which are films intended to be transparent in the inactivated state but reversibly darkenable to a light-absorbing state by the action of incident light. Photochromic films of various configurations have been described by Brewer et al. in French Pat. No. 2,236,196, by Gliemeroth et al. in U.S. Pat. No. 3,875,321, by Plumat et al. in U.S. Pat. No. 3,512,869, and by Perveyev et al. in the Soviet Journal of Optical Technology, February, 1972, pp. 117-118.
In photochromic films, the feature which is desired is that of rapid and complete thermal fading of the darkened film to a generally clear state after irradiation with activating light is terminated. In contrast, photosensitive films intended for optical information storage should resist thermal fading so that variations in optical behavior (e.g., optical density) induced by irradiating the films will be relatively permanent.
For the optical storage of information in digital form, a thin optical recording medium which is optically alterable to a highly dichroic and birefringent state is desired. Although some of the known photosensitive glasses and silver halide photographic emulsions can provide relatively strong birefringence and dichroism, they are generally thicker than would be desired for efficient information storage. A focused laser beam is the best source for recording optical information in compact digital form, permitting spot sizes on the order of 1 micron or less. When films substantially thicker than about 2 microns are used, losses in spot resolution significantly limit the density of information storage.
While thin photosensitive films do not impose such limitations on resolution, the levels of dichroism and birefringence which have been observed such thin films produced in the prior art are somewhat limited. High levels of dichroism and birefringence are advantageous for information retrieval from such films because image contrast may be enhanced by viewing in transmitted light between crossed polarizers.
It is therefore a principal object of the present invention to provide photosensitive films for the optical storage of information which are limited in thickness and yet alterable to a highly dichroic and birefringent state by irradiation with linearly polarized light.
It is a further object of the invention to provide methods for producing photosensitive films with improved optical information storage behavior.
Further objects and advantages of the invention will become apparent from the following description thereof.